1. Field of the Invention
The present invention relates to a high-frequency amplifying device which is used in, for example, a mobile communication terminal to amplify a high-frequency signal.
2. Description of the Rior Art
FIG. 1 is a block diagram showing a configuration of conventional high-frequency amplifying device described in, for example, “Intelligent RF Power Module Using Automatic Bias Control (ABC) System for PCS CDMA Applications” (Sato et al., IEEE MTT-S Int. Microwave Symp. Dig. p.p. 201-204, 1998). In the figure, reference numeral 201 indicates an input terminal, 202-1 indicates a first-stage transistor for amplifying a high-frequency signal, and 202-2 indicates a second-stage transistor for amplifying the high-frequency signal. 203-1 indicates a bias feed circuit such as a distributed constant circuit, a resistor, an inductor, a capacitor or the like for biasing the input of transistor 202-1, and 203-2 indicates a bias feed circuit for biasing the input of transistor 202-2. 204-1 indicates a pull-up circuit provided on the output side of transistor 202-1, 204-2 indicates a pull-up circuit provided on the output side of transistor 202-2, and 205 indicates an output terminal, respectively.
In addition, reference numeral 211 indicates a transistor for detecting an input signal, 212 indicates a pull-up circuit on the output side of transistor 211, and 213 indicates a comparator for comparing a reference voltage generated by a reference voltage source 214 and a voltage of signal detected by the transistor 211. 214 indicates a reference voltage source for generating a predetermined reference voltage, and 215 indicates a variable voltage source for applying a voltage corresponding to the result of comparison by the comparator 213 to each bias feed circuits 203-1 and 203-2.
The operation will next be explained.
A-high-frequency signal supplied via the input terminal 201 is amplified by the transistors 202-1 and 202-2 and the post-amplification high-frequency signal is output through the output terminal 205.
On the other hand, the transistor 211 detects the high-frequency signal supplied via the input terminal 201 and supplies a post-detection signal to the comparator 213. The comparator 213 compares a reference voltage generated by the reference voltage source 214 and a voltage of signal detected by the transistor 211 and supplies a signal (e.g., a signal of 0 or 1) indicative of whether the voltage of post-detection signal is higher than the reference voltage, to a variable voltage source 215. The variable voltage source 215 changes the voltage to apply to each bias feed circuits 203-1 and 203-2 according to whether or not the voltage of input signal is higher than the reference voltage.
Thus the biases to be applied to the transistors 202-1 and 202-2 are changed according to the voltage (power) of input signal to reduce power consumption at low output power.
However, because the conventional high-frequency amplifying device is constructed as described above, it has been accompanied by problems that a variation in gain at their changing is large and, for example, further reduction in power consumption is difficult, since the biases to be applied to the transistors are changed discontinuously based on whether or not the voltage of input signal is higher than the reference voltage. There was a possibility that when the variation in gain at the changing was large, a shift occurred in the phase of a signal, thus causing defective conditions upon detection. When a system, such as a W-CDMA (Wideband Code Division Multiple Access) system which is placed under strict constraints on a gain fluctuation band width, is used in a communication apparatus or the like, it has been difficult to meet such constraints when the gain variation at the changing is large.
The present invention has been made to solve the foregoing problems. Therefore the present invention aims to provide a high-frequency amplifying device comprising a high-frequency amplifying unit having a plurality of the amplifying elements for amplifying the input high-frequency signal; a measuring circuit for measuring amplitude of the input high-frequency signal; and a bias control circuit for continuously controlling a bias applied to each amplifying elements according to value of the amplitude measured by the measuring circuit, whereby a steep gain variation developed according to a change in the amplitude of input high-frequency signal can be suppressed.